In the electrodeposition of tin and lead, many methods have been proposed which have, in general, evolved towards methods based on depositing these two metals as alloys from what is known as a fluoride or a fluoroborate bath. These alloys so deposited provide all the advantages for soldering associated with tin such as in printed circuitry and other circuits wherever a tin overplate is needed to achieve better soldering properties. Further, fluoride baths are used to obtain circuit boards with 93 percent tin to 7 percent lead deposits as a replacement for pure tin deposits. The later are subject to whisker growth and thus cause short circuiting across closely spaced circuit patterns. Because of these advantages, the fluoride and fluoroborates have substantially dominated the tin-lead plating bath without serious competition being offered by any other bath for depositing tin-lead alloys.
However, various disadvantages have become increasingly evident when using the fluoride and fluoroborate bath for depositing tin-lead alloys. First, the above-mentioned waste treatment is very complicated for fluorides and fluoroborates and the treatment requirements have become increasingly strict to a point where the economic benefits are now questionable.
Inasmuch as the plating equipment is severly corroded by the fluoride and fluoroborate bath, the various shortcomings have prompted a search for a replacement bath having all the advantages but as few disadvantages as possible.
Although various methods have been proposed for coating electrodeposition lead such as lead and antimony in U.S. Pat. No. 2,718,494 or tin and antimony in U.S. Pat. No. 2,825,683 as well as various coatings of lead at a pH of about 8 and tin at a pH of about 1, for example in U.S. Pat. No. 2,919,233, the use of a method for depositing tin-lead alloys in controlled conditions has not been disclosed from a bath of the present composition.
Other bath solutions which have been disclosed such as in U.S. Pat. No. 3,039,942 provide for a complicated method of electrodepositing from solutions which are based on pyrophosphates but are not of the composition disclosed in the present process. These compositions are subject to rapid variations in deposit composition and are very hard to control because powdery deposits are often produced. Other baths which describe nonanalogous tin alloys have been reported in Bull. India Sect., Electrochem. Soc., Vol. 9, pp. 13 to 14, (1960).